MINERAL GEOCHEMISTRY OF CRETACEOUS AND EOCENE PLUTONIC ROCKS FROM THE OKANOGAN HIGHLANDS, NORTHEASTERN WASHINGTON STATE

Biotite and felspar were chemically analyzed from three plutonic rocks from the Okanogan Highlands, WA, using the EPMA at the Florida Center for Analytical Electron Microscopy, Florida International University. These minerals are from an Eocene tonalite from the Swawilla Granodiorite, a Cretaceous monzogranite from the Midnight Mine pluton, and a Cretaceous granodiorite from the Conconully pluton. Iron in the biotite is calculated as total Fe²⁺. EPMA analysis included 7 biotite and 8 feldspar from the Swawilla Granodiorite, 6 biotite and 6 feldspar from the Midnight Mine pluton, and 7 biotite and 2 feldspar from the Conconully pluton. Biotite Fe# and ΣAl range from 0.49 to 0.57 and 2.61 to 2.69 apfu in the Swawilla Granodiorite sample, 0.53 to 0.61 and 2.80 to 2.89 apfu in the Midnight Mine pluton sample, and 0.47 to 0.60 and 2.60 to 2.84 apfu in the Conconully pluton sample. The strong brown pleochroism, textures, and chemistry of the biotite from all samples suggest they are primary. Plagioclase from the Swawilla Granodiorite is primarily andesine (An_34-30), and the Midnight Mine pluton is primarily oligoclase (An_31-25). One analyzed alkali feldspar from the Midnight Mine pluton sample is Or₉₂ and two analyzed alkali feldspar from the Conconully pluton sample are Or₉₄ and Or₉₁.

The biotite analyses plot within calc-alkaline fields on discrimination diagrams. Biotite thermometry of Luhr et al. (1984) and barometry of Uchida et al. (2007) where used to estimate crystallization temperatures and pressures. The biotite yields temperatures and pressures of 600° ± 6 C and 1.53 ± 0.07 kb for the Swawilla Granodiorite sample, 596° ± 2 C and 2.08 ± 0.09 kb for the Midnight Mine pluton sample, and 679° ± 31 C and 1.64 ± 0.20 kb for the Conconully pluton sample. Oxygen fugacity using the equilibrium expression of Wones (1989) was calculated using the biotite temperatures and pressures. These yield log ƒO₂ of -20.45 ± 0.24 for the Swawilla Granodiorite sample, -20.62 ± 0.07 for the Midnight Mine pluton sample, and -17.53 ± 1.14 for the Conconully pluton sample. This data suggests that these samples crystalized in arc settings at relatively shallow depths. These estimated pressures suggest no appreciable uplift occurred between the Cretaceous plutons and the Eocene Swawilla Granodiorite crystallization.